1. Field of the Invention
This invention relates to catalytic hydrocracking. More particularly, this invention is concerned with hydrocracking of polynuclear aromatic containing feedstocks with a catalyst exemplified by small crystal size ZSM-5 associated with nickel-tungsten.
2. Description of the Prior Art
The hydrocracking of hydrocarbons to produce lower boiling hydrocarbons, and in particular, hydrocarbons boiling in the motor fuel range, is an operation upon which a vast amount of time and effort has been spent in view of its commercial significance. Hydrocracking catalysts usually comprise a hydrogenation-dehydrogenation component deposited on a acidic support such as silica-alumina, silica-magnesia, silica-zirconia, alumina, acid treated clays, zeolites and the like.
Crystalline zeolites have been found to be particularly effective in the catalytic hydrocracking of a gas oil to produce motor fuels and such has been described in many U.S. patents including Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418.
A catalytic hydrocracking process utilizing a catalyst comprising a zeolite dispersed in a matrix of other components such as nickel, tungsten and silica-alumina is described in U.S. Pat. No. 3,617,498. A hydrocracking catalyst comprising a zeolite and a hydrogenation-dehydrogenation component such as nickel-tungsten-sulfide is recited in U.S. Pat. No. 4,001,106. In U.S. Pat. No. 3,758,402, a hydrocracking process is disclosed wherein the catalyst comprises a large pore zeolite such as zeolite X or Y and a smaller pore zeolite such as ZSM-5 which may have a hydrogenation/dehydrogenation component such as nickel-tungsten associated with at least one of the zeolites. Hydrocarbon conversion utilizing a catalyst comprising a zeolite, such as ZSM-5, having a zeolite particle diameter in the range of 0.005 micron to 0.1 micron and in some instances containing a hydrogenation/dehydrogenation component is related in U.S. Pat. No. 3,926,782. The hydrocracking of lube oil stocks employing a catalyst comprising hydrogenation components and a zeolite such as ZSM-5 is disclosed in U.S. Pat. No. 3,755,145.
Whereas a great amount of attention has been given to hydrocracking petroleum gas oils, much less emphasis has been devoted to hydrocracking polynuclear aromatic containing feedstocks such as FCC cycle oils and coal derived liquids. Such polynuclear aromatic stocks require saturation and thus increased hydrogen consumption during processing in order to produce a suitable liquid product. U.S. Pat. No. 3,523,886 discloses a process for making liquid fuel from coal by solvent extraction which involves catalytic hydrocracking.
Hydrocracking generally requires a clean feedstock, or alternatively, due to the large heteroatom content of many feedstocks, hydrocracking frequently must be preceded by a pretreatment stage. It would be very advantageous to have a system which would be able to both pretreat and hydrocrack in one operation.